Thread: pipes implementation in C

Originally Posted by khdani

Hello,
I'm trying to write a simple shell in C.
How do I implement redirection (< , >) and pipes '|' in C ?
Thank You

You will need pipe(), dup() and/or dup2(), open(), close(), fork(), and exec() in some magical combination

There are a lot of references to this on the web. If you start there, I think we'd be happy to work on specific questions with you.

that's a specific question how do i implement '>'
for example "ls -l >out.txt"
my shell can execute commands like "ls -l" or any other command.
i don't think i need pipe() for |, it's probably implemented with files...

Originally Posted by khdani

that's a specific question how do i implement '>'
for example "ls -l >out.txt"

The problem is, none of us really have time to write up the several hundred word response that would completely describe how to do that.

Here's some pseudocode, but I'm not going to fill in all the blanks

Code:

fd = open("out.txt")
if fork() == parent:
    close(fd)
    wait for the child
else:
    close(1)
    dup(fd)
    exec("/bin/ls")

In other words, the shell opens the output file. Then it forks itself to create a child. The child closes its stdout descriptor, and calls dup() to replace that with the fd of the open file. Then it executes the ls program. The parent, meanwhile, closes the fd (since it doesn't need it) and waits for the child to finish running.

If you want more details, I'll trade you -- provide some code.

Originally Posted by khdani

i don't think i need pipe() for |, it's probably implemented with files...

I don't know why you'd think that. That's exactly where you would use pipe().